The invention relates to a method for operating an actuating drive for controlling fittings and of the type wherein an electric motor that is fed from a frequency converter moves a fitting with a predetermined torque through a self-locking gearbox and more particularly to controlling the temperature of the drive.
Actuating drives are used for process automation in a process installation, in order to open and to close valves, flaps and the like.
Such an actuating drive essentially comprises an electric motor, which moves a fitting with a predetermined torque via a self-locking gearbox. If the electric motor is overloaded, the motor windings are heated unacceptably, and this can lead to damage or destruction of the motor windings. Such overloads occur regularly if the fitting is jammed and the force disconnection fails at the same time.
Furthermore, the problem that arises in particular with high-power actuating drives is that self-locking gearboxes become severely heated during continuous operation, due to their low efficiency. The thermal coupling between the electric motor and the gearbox results in the electric motor being heated further.
In order to protect motor windings, it is known from the publication from the Auma Co. xe2x80x9cDrehantriebe fur Steuerbetriebxe2x80x9d [Rotary drives for control operation], Issue 2.96, for the circuit for feeding the electric motor to be interrupted as soon as the winding temperature exceeds a predetermined limit value. However, this procedure is subject to the disadvantage that, throughout the entire period during which an excessive temperature is identified, the fitting is locked in its most recently assumed position, no longer reacts to control commands, and the sequence in the surrounding process installation is thus interfered with.
The invention is thus based on the object of specifying a method for operating an actuating drive for controlling fittings, which allows the actuating drive to be protected against unacceptable, destructive heating, and nevertheless allows operation of the surrounding process installation to be maintained, despite the excessive temperature.
The invention is based on an actuating drive whose electric motor is connected to a frequency converter. Such frequency converters are generally known per se and are described, for example, in DE 42 35 223. During correct use, the frequency and the output voltage of the frequency converter are set in accordance with the control object from the surrounding process installation. In this case, the rotation speed of the electric motor is set by the frequency, and the torque which is required to move the given fitting in accordance with the control task is set by the output voltage at the given frequency.
Depending on the load on the actuating drive, an amount of heat is produced in the actuating drive which is caused essentially by mechanical friction and electrical power losses. The amount of heat produced is greater than the amount of heat that can be dissipated via the surface of the actuating drive to the environment. This leads to an increase in the temperature of the actuating drive.
According to the invention, the temperature of the actuating drive is monitored, and the rotation speed of the electric motor is reduced, as a function of the measured temperature, while the torque is kept constant. To this end, the motor voltage emitted from the frequency converter is reduced in accordance with a characteristic which can be predetermined, virtually in proportion to the stator frequency.
If the torque is kept constant, the power supplied to the gearbox falls in proportion to the reduction in the drive rotation speed. The power losses which lead to heating of the gearbox fall in accordance with the efficiency xcex7, which is governed by the gearbox design and, for self-locking gearboxes, is always xcex7xe2x89xa60.5.
The drive rotation speed is set so that the gearbox temperature caused by the power losses in the gearbox remains below the predetermined limit value. The actuating drive is advantageously protected against excessive temperature increases, while nevertheless remaining ready to operate during periods when the temperature is raised. This avoids disturbances in the operational sequence of the surrounding process installation.